A known apparatus is described, for instance, in U.S. Pat. No. 4,341,310. This document discloses a so-called continuous jet printer for printing materials using a first droplet ejection system arranged to generate a continuous stream of first droplets from a fluid jetted out of an outlet channel. During the exit of the fluid through an outlet channel, a pressure regulating mechanism provides, with a predetermined regularity, variations in the pressure of the viscous fluid adjacent the outflow opening. This leads to the occurrence of a disturbance in the fluid jet flowing out of the outflow opening. This disturbance leads to a constriction of the jet which in turn leads to a breaking up of the jet into drops. This yields a continuous flow of egressive drops with a uniform distribution of properties such as dimensions of the drops.
The publication further discloses a second droplet ejection system arranged to generate second droplets for colliding the second droplets into the first droplets so as to selectively deflect the first droplets from a predefined printing trajectory. The second droplet ejection system is of a continuous nature and uses polar fluids to deflect a second stream of droplets into the continuous stream of the first droplet ejection system.